REPORTING FROM NEW DELHI — A major earthquake struck off the coast of Indonesia Wednesday afternoon with a reported magnitude of 8.6, sparking tsunami warnings for Australia and coastal nations across Southeast Asia, central Asia and parts of the Arabian Gulf.

Experts cautioned that a tsunami warning signals a potential for an oversized tidal wave, not necessarily that it is imminent.

According to the U.S. Geological Survey, the powerful quake was centered 20 miles beneath the ocean floor around 308 miles from the provincial capital of the Indonesian island of Aceh.

Experts said early indications were that the quake, which struck around 2:30 p.m. local time, was caused by a horizontal shift in the earth’s plates of a type less likely to cause severe tsunamis than vertical shifts, which tend to displace larger volumes of water.

Footage showed terrified Indonesians pouring into the street, trying to make calls on their cell phones and hugging each other in terror, some going back into buildings to find lost colleagues.

An enormous 9.1-magnitude earthquake off Indonesia in late 2004 in roughly the same area triggered a tsunami in the Indian Ocean that killed an estimated 230,000 people, nearly three quarters of them in Aceh.

Residents as far away as Thailand and southern India said they felt the temblor.

“Buildings were moving about, twice in the space of 10 minutes,” said Lakshmi Vijayakumar, founder of a suicide hotline in Chennai, the capital of India’s southern Tamil Nadhu state, adding that they’d received an increase in panicked calls.

“People were scared,” she added. “Many came into the streets, particularly from high-rise buildings. Glass has shattered.” She added that her cousin in a high-rise building along the shore was told to evacuate against a possible tsunami they she was told might hit at 5 p.m. “But so far I don’t think there’s too much property damage,” she said.